Mode-locked lasers are characterized by the locking of multiple axial modes in the laser cavity, thus resulting in resonant, ultrashort pulsing phenomena. Mode-locked lasers have become commercially and scientifically successful, impacting medical imaging, two-photon microscopy, femtosecond chemistry, micro-machining, surgery, and fusion research, to name a few example fields. Fiber-based mode-locked lasers are particularly interesting due to the numerous inherent advantages of the optical fiber platform. Recent trends have shown that these fiber-based lasers may eventually achieve competitive performance with their solid-state counterparts, thus potentially shifting the field of ultra-fast, high-power lasers to fiber-based technologies. Closing the order of magnitude performance gap between fiber and solid-state lasers will involve obtaining the ability to control and optimize laser cavity output energy and pulse width.
One of the most prolific and dominant fiber-based mode-locking lasers demonstrated to date involves a linear polarizer and a number of waveplates to achieve saturable absorption via nonlinear polarization rotation (NPR). It remains challenging and expensive to find high-energy, single-pulse solutions in the multiple-NPR case. Moreover, even if mode-locking is achieved, it may be destroyed by changes to the birefringence, which often varies throughout the day based on changes in ambient conditions and may change abruptly if the laser system is physically perturbed.
What is needed are techniques for quickly and efficiently tuning a mode-locked laser or other multi-input complex dynamic system in order to automatically obtain optimal performance.